1) Field of the Invention
This invention relates generally to the fabrication of capacitors in dynamic random access memory (DRAM) cells and more particularly to a structure and a method for fabricating capacitors with a large capacitance.
2) Description of the Prior Art
In dynamic semiconductor memory storage devices it is essential that storage node capacitor cell plates be large enough to retain an adequate voltage level in spite of parasitic capacitances and noise that may be present during circuit operation. As is the case for most semiconductor integrated circuitry, circuit density is continuing to increase at a fairly constant rate. The issue of maintaining storage node capacities is particularly important as the density of DRAM cells continues to increase for future generations of memory devices. The ability to densely pack storage cells while maintaining required storage capabilities is a crucial requirement of semiconductor manufacturing technologies if future generations of expanded memory array devices are to be successfully manufactured.
The array of storage cells on a dynamic random access memory (DRAM) chip is one circuit element experiencing electrical limitations. These individual DRAM storage cells, usually consisting of a single metal-oxide-semiconductor field-effect-transistor (MOSFET) and a single capacitor are used extensively in the electronic industry for storing data. A single DRAM cell stores a bit of data on the capacitor as electrical charge. The decrease in cell capacitance caused by reduced memory cell area is a serious obstacle to increasing packing density in dynamic random access memories (DRAMs). Thus, the problem of decreased cell capacitance must be solved to achieve higher packing density in a semiconductor memory device, since decreased cell capacitance degrades read-out capability and increases the soft error rate of the memory cell as well as consumes excessive power during low-voltage operation by impeding device operation.
Generally, in a 64 MB DRAM having a 1.5 .mu.m.sup.2 memory cell area employing an ordinary two dimensional stacked capacitor cell, sufficient cell capacitance cannot be obtained even though a higher dielectric constant material, e.g., tantalum oxide (Ta.sub.2 O.sub.5), is used. Therefore, stacked capacitors having a three-dimensional structure have been suggested to improve cell capacitance. Such stacked capacitors include, for example double-stacked, fin-structured, cylindrical, spread-stacked, and box structured capacitors. In order to increase the surface area of the capacitor, there have also been proposed methods of forming a capacitor with a pin structure extending throughout a multi-layer structure of the capacitor to connect the layers with one another and a method of forming a capacitor using a hemispherical grain polysilicon (HSG) process using polysilicon grains.
Workers in the art are aware of the limitation of capacitors and have attempted to resolve them. U.S. Pat. No. 5,364,809 (Kwon et al.) teaches a method of fabricating a multi-chamber type capacitor. A stacked capacitor having a concave area is formed and then oxide spacers are formed in the concave area. U.S. Pat. No. 5,266,513 (Frazan et al.) teaches a method to form a stacked multi-fingered cell capacitor and extension over an adjacent cell. U.S. Pat. No. 5,451,539 (Ryou) teaches a method of forming a double wall cylindrical capacitor. Ryou forms the two upright walls as spacers on an oxide cylinder. However, many of the prior art methods require substantially more processing steps or/and planar structures that make the manufacturing process more complex and costly. Therefore, it is very desirable to develop processes that are as simple as possible and maximize the capacitance per unit area. There is a challenge to develop methods of manufacturing these capacitors that minimize the manufacturing costs and maximize the device yields. In particular, there is a challenge to develop a method which minimizes the number of photoresist masking operations and provides maximum process tolerance to maximize product yields.